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What are solar flares?

A solar flare is basically a giant explosion on the surface of our Sun which occurs when magnetic field lines from sunspots tangle and erupt. A solar flare is defined as a sudden, rapid, and intense variation in brightness. A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. Material is heated to many millions of degrees in just minutes and radiation is emitted across virtually the entire electromagnetic spectrum, from radio waves at the long wavelength end, through optical emission to X-rays and gamma rays at the short wavelength end. The amount of energy released is equivalent to millions of nuclear bombs exploding all at the same time! Solar flares are an often occurrence when the Sun is active in the years around solar maximum. Many solar flares can occur on just one day during this period! Around solar minimum, solar flares might occur less than once per week. Large flares are less frequent than smaller ones.

A solar flare as captured by NASA's Solar Dynamics Observatory

Image: A spectacular solar flare as seen by the NASA's Solar Dynamics Observatory in the 193 Ångström wavelength.

Some (mostly stronger) solar flares are known to produce so called coronal mass ejections which can lead to geomagnetic storms when they are directed towards Earth.

The classification of solar flares

Solar flares are classified as A, B, C, M or X according to the peak flux (in watts per square metre, W/m2) of 1 to 8 Ångströms X-rays near Earth, as measured by XRS instrument on-board the GOES-15 satellite which is in a geostationary orbit over the Pacific Ocean. The table below shows us the different solar flare classes:

Class W/m2 between 1 & 8 Ångströms
A <10-7
B ≥10-7 <10-6
C ≥10-6 <10-5
M ≥10-5 <10-4
X ≥10-4

Each X-ray class category is divided into a logarithmic scale from 1 to 9. For example: B1 to B9, C1 to C9, etc. An X2 flare is twice as powerful as an X1 flare, and is four times more powerful than an M5 flare. The X-class class category is slightly different and doesn't stop at X9 but continues on. Solar flares of X10 or stronger are sometimes also called ''Super X-class solar flares.''

A & B-class solar flares

The A & B-class are the lowest class of solar flares. They are very common and not very interesting. The background flux (amount of radiation emitted when there are no flares) is often in the B-range during solar maximum and in the A-range during solar minimum.

C-class solar flares

C-class solar flares are minor solar flares that have little to no effect on Earth. Only C-class solar flares which are long in duration might produce a produce a coronal mass ejection but they are usually slow, weak and rarely cause a significant geomagnetic disturbance here on Earth. The background flux (amount of radiation emitted when there are no flares) can be in the lower C-class range when a complex sunspot region inhabits the Earth-facing solar disk.

M-class solar flares

M-class solar flares are what we call the medium large solar flares. They cause small (R1) to moderate (R2) radio blackouts on the daylight side of the Earth. Some eruptive M-class solar flares can also cause solar radiation storms. Strong, long duration M-class solar flares are likely candidates to launch a coronal mass ejection. If the solar flare takes place near the center of the Earth-facing solar disk and launches a coronal mass ejection towards our planet, there is a high probability that the resulting geomagnetic storm is going to be strong enough for aurora on the middle latitudes.

X-class solar flares

X-class solar flares are the biggest and strongest of them all. On average, solar flares of this magnitude occur about 10 times a year and are more common during solar maximum than solar minumum. Strong to extreme (R3 to R5) radio blackouts occur on the daylight side of the Earth during the solar flare. If the solar flare is eruptive and takes place near the center of the Earth-facing solar disk, it could cause a strong and long lasting solar radiation storm and release a significant coronal mass ejection that can cause severe (G4) to extreme (G5) geomagnetic storming at Earth.

X-class solar flare as seen by NASA's Solar Dynamics Observatory in the 131 Ångström wavelength

Image: An X-class solar flare as seen by NASA's Solar Dynamics Observatory in the 131 Ångström wavelength.

So what's above X9? The X-class continues after X9 instead of getting a new letter and these solar flares are often referred to as ''Super X-class'' solar flares. Solar flares that reach or even surpass the X10 class are however very rare and occur only a few times during a solar cycle. It is actually a good thing that these powerful solar flares do not occur so often as the consequences on Earth could be severe. The coronal mass ejections which can be launched by such solar flares are known to be able to cause issues with our modern technology like satellites and power lines.

One thing to note with super X-class flares is that an X20 solar flare is not 10 times as strong as an X10 solar flare. An X10 solar flares equals an X-ray flux of 0.001 Watts/m2 while an X20 solar flare equals 0.002 Watts/m2 in the 1-8 Ångstrom wavelength.

The largest solar flare ever recorded since satellites started to measure them in 1976 was estimated to be an X28 solar flare which occurred on November 4th, 2003 during Solar Cycle 23. The XRS long channel on the GOES-12 satellite was saturated at X17 for 12 minutes by the intense radiation. A later analysis of the available data yield an estimated peak flux of X28 however there are scientists who think that this solar flare was even stronger than X28. A good thing for us was that the sunspot group which produced this solar flare had already rotated largely of the Earth-facing solar disk when the X28 solar flare occurred. A thing to note is that there has not been a solar flare that saturated the XRS channels on GOES-15 as of March 2017 but it is expected that it will saturate at about the same flux levels.

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